Phthalic anhydride is conventionally produced either by the oxidation of naphthalene with molecular oxygen over a fixed or fluid bed catalyst or by the oxidation of ortho-xylene with molecular oxygen over a fixed bed catalyst. These oxidation reactions are highly exothermic, and, therefore, it is desirable to conduct them in a fluidized bed reactor where high heat transfer rates may be achieved to obtain good temperature control. In a fixed bed system, temperature control is difficult and is achieved by using a multiplicity of small diameter tubes contained within a shell in which a coolant such as molten NaNO.sub.3 /NaNO.sub.2 is circulated. The heat of reaction is removed from the coolant by circulation through an external heat exchanger. This system is complex and costly.
While it has been found, in the case of naphthalene oxidation, that equally satisfactory results can be achieved in either fixed or fluid beds, such is not the case of o-xylene oxidation. Attempts to conduct the o-xylene oxidation reaction in a fluid bed have been unsuccessful because a suitable fluidizable catalyst has not heretofore been readily available, and because the reaction is best carried out in a short residence time-plug flow regime, conditions which cannot be attained in a conventional fluid bed reactor.
Several moving bed type reactor systems have been described in the art for the purpose of retaining the good heat transfer characteristics of the fluid bed system, while at the same time attempting a plug flow regime.
U.S. Pat. No. 3,600,440 to Shell describes a compact moving bed system. In this system temperature control is good and backmixing of gases can be minimized, but movement of the solids is difficult and impractical. Further, while it is suggested that the explosion hazard which exists when operating in the flammability region can be eliminated because of the good heat transfer, this would be a very dangerous practice. This patent, while mentioning the oxidation of naphthalene, does not teach that the system is applicable to o-xylene oxidation.
U.S. Pat. No. 3,565,919 describes a fluidized bed catalyst for converting o-xylene to phthalic anhydride. Though a broad range of particle size is claimed, the exemplified particles have mean diameters in the range of 0.1 to 0.4 mm. Such process, wherein the catalyst particles as shown in the examples are fluidized vigorously in the presence of porcelain spheres, permits backmixing and would not achieve the desired plug flow.
U.S. Pat. No. 2,526,689 to Standard Oil describes a dense phase fluidized bed transport system which utilizes intermediate size particles (40 to 80 mesh). The operating regime in this system would result in significant backmixing of both catalyst particles and reacting gases. Furthermore, the catalysts described are not practical, i.e., they have poor physical strength and reported conversion of o-xylene to phthalic anhydride is impractically low.
A reactor system which retains the good heat transfer characteristics of the fluidized bed system and the plug flow characteristics of the fixed bed system is the dilute phase transport reactor system. In this system, particles size is small enough and gas velocity is high enough to ensure that particles and gases are in essentially plug flow motion in the reactor. Wainwright and Hoffman, in Chemical Reaction Engineering II, Advances in Chemistry, V. 133 (1974), pages (670-685, evaluated such a system for the oxidation of o-xylene to phthalic anhydride. The selectivity to phthalic anhydride was very poor, however.
Furthermore, the transported bed reactor taught is also unsatisfactory because complicated equipment is required, particularly in the product recovery system. This complication is largely because it was believed that, for safety considerations, the oxidation should be performed at oxygen concentrations below the flammability limit. Simply stated, in order to avoid the danger of explosion, it is generally understood that the reactor should not contain more than 1 mole % or organic constituents. Because of this required dilution, only very low concentrations of the reaction product appear in the reaction effluent and large quantities of inerts (primarily nitrogen) and unreacted effluent and large quantities of inerts (primarily nitrogen) and unreacted oxygen pass through the reactor and the recovery system. With the other alternative, that of operating with a feed composition above the upper flammability limit, i.e., 6 to 10 mole % organics, only about 50% conversion is achieved and a large amount of xylene has to be recovered and recycled. Still another factor is that during the course of the reaction the oxygen concentration must be maintained above approximately 5 mole % (i.e., a partial pressure of 0.05 atm.), because, at lower concentrations, catalyst activity is not maintained. The ideal objective of 100% conversion of o-xylene with the introduction of a minimum of inerts could only be obtained with air as the oxidant at approximately 3 mole % organics in the feed, a concentration directly in the middle of the flammable region!
Other references relating to the plug flow catalytic reaction include P. H. Calderbank et al., "The Prediction of the Performance of Packed-Bed Catalytic Reactors in the Air-Oxidation of o-Xylene," Chemical Engineering Science, 1977, Vol. 32, pp. 1435-1443, and M. S. Wainwright et al., "The Oxidation of Ortho-xylene on Vanadium Pentoxide Catalysts," The Canadian Journal of Chemical Engineering, vol. 55, October 1977, pp. 552-5634. To the extent operation in the explosive limit is taught in these references, the process is too dangerous to practice commercially. To the extent these references show the use of o-xylene concentrations of about 1 mole %, such process suffers from the drawback of requiring the costly equipment for the processing of large volumes of inert gas.
Gulf Research & Development Co., U.S. Pat. No. 4,102,914, shows a "fast fluidization" reactor for the ammoxidation of propylene with stagewise oxygen feed, but its teaching is solely restricted to preparing acrylonitrile.